Signaling involving the Transforming Growth Factor β (TGF-β) superfamily of ligands is central to a wide range of cellular processes, including cell growth, differentiation, and apoptosis. TGF-β signaling involves binding of a TGF-β ligand to a type II receptor (a serine/threonine kinase), which recruits and phosphorylates a type I receptor. The type I receptor then phosphorylates a receptor-regulated SMAD (R-SMAD; e.g., SMAD1, SMAD2, SMAD3, SMAD5, SMAD8 or SMAD9), which binds to SMAD4, and the SMAD complex then enters the nucleus where it plays a role in transcriptional regulation. The TGF superfamily of ligands includes two major branches, characterized by TGF-β/activin/nodal and Bone Morphogenetic Proteins (BMPs).
Signals mediated by bone morphogenetic protein (BMP) ligands serve diverse roles throughout the life of vertebrates. During embryogenesis, the dorsoventral axis is established by BMP signaling gradients formed by the coordinated expression of ligands, receptors, co-receptors, and soluble antagonists. Excess BMP signaling causes ventralization, an expansion of ventral at the expense of dorsal structures, while diminished BMP signaling causes dorsalization, an expansion of dorsal at the expense of ventral structures. BMPs are key regulators of gastrulation, mesoderm induction, organogenesis, and endochondral bone formation, and regulate the fates of multipotent cell populations. BMP signals also play critical roles in physiology and disease, and are implicated, for example, in primary pulmonary hypertension, hereditary hemorrhagic telangiectasia syndrome, fibrodysplasia ossificans progressiva, and juvenile polyposis syndrome among others.
The BMP signaling family is a diverse subset of the TGF-β superfamily. Over twenty known BMP ligands are recognized by three distinct type II (BMPRII, ActRIIa, and ActRIIb) and at least three type I (ALK2, ALK3, and ALK6) receptors. Dimeric ligands facilitate assembly of receptor heteromers, allowing the constitutively-active type II receptor serine/threonine kinases to phosphorylate type I receptor serine/threonine kinases. Activated type I receptors phosphorylate BMP-responsive (BR-) SMAD effectors (SMADs 1, 5, and 8) to facilitate nuclear translocation in complex with SMAD4, a co-SMAD that also facilitates TGF signaling. In addition, BMP signals can activate intracellular effectors such as MAPK p38 in a SMAD-independent manner. Soluble BMP antagonists such as noggin, chordin, gremlin, and follistatin limit BMP signaling by ligand sequestration.
A role for BMP signals in regulating expression of hepcidin, a peptide hormone and central regulator of systemic iron balance, has also been suggested. Hepcidin binds and promotes degradation of ferroportin, the sole iron exporter in vertebrates. Loss of ferroportin activity prevents mobilization of iron to the bloodstream from intracellular stores in enterocytes, macrophages, and hepatocytes. The link between BMP signaling and iron metabolism represents a potential target for therapeutics.
Given the tremendous structural diversity of the BMP and TGF-β superfamily at the level of ligands (>25 distinct ligands at present) and receptors (three type I and three type II receptors that recognize BMPs), and the heterotetrameric manner of receptor binding, traditional approaches for inhibiting BMP signals via soluble receptors, endogenous inhibitors, or neutralizing antibodies are not practical or effective. Endogenous inhibitors such as noggin and follistatin have limited specificity for ligand subclasses. Single receptors have limited affinity for ligand, whereas ligand heterotetramers exhibit rather precise specificity for particular ligands. Neutralizing antibodies are specific for particular ligands or receptors and are also limited by the structural diversity of this signaling system.
Thus, there is a continuing need for pharmacologic agents that antagonize BMP signaling pathways and that can be used to manipulate these pathways in therapeutic or experimental applications.